The present invention relates to analytical chemistry and, more particularly, to gas chromatography. A major objective of the invention is to provide an improved flash injector for a gas chromatography system.
Analytical chemistry has advanced human and environmental health by permitting the components of, for example, medical and environmental samples to be separated for detection, identification, and quantification. Gas chromatography, an increasingly important separation method, separates vaporized analyte components according to their different partitionings between a mobile and a stationary phase.
In a typical gas chromatography system, the tubular column is packed or internally coated with material that differentially absorbs analyte components. As analyte molecules flow past the absorbent material, they are alternately absorbed by it and adsorbed back into the carrier gas. Each component achieves an equilibrium at which the rate of absorption equals the rate of adsorption so that the percentage of molecules that are absorbed remains constant. The higher the percentage of a component's molecules that are absorbed, the slower the component migrates down the column. Thus, the components progressively separate according to their partitioning constants. Once separated, the components can be individually detected and/or collected for further analysis.
The molecules associated with a particular component do not all elute from the column at the same instant; instead, the molecules are distributed around a peak. If the column is unable to provide sufficient separation (spatial resolution) between two components, the distributions overlap. The overlap makes it more difficult to quantify the components and to collect them individually.
One method of increasing spatial resolution is to use a longer column so components have more time to separate. However, the gain in resolution is partially offset by the additional spreading that occurs in the extra time. Moreover, the longer separation time decreases analytical throughput. The optimizing column length is a practical tradeoff between increased separation and these negative results. Given that column length is readily optimized, further improvements require other approaches to improving spatial resolution.
Decreasing the inner diameter of the column can increase spatial resolution by shortening the average radial diffusion distance, which, in turn, reduces the standard deviation of elution time. Peaks are narrower and thus less prone to overlap. The additional resolution obtainable using a narrower column can be traded for a shorter column to reduce analysis time and increase analysis throughput.
The decreased column diameter limits the amount of analyte that can be injected for a given plug length. The amount of analyte is further limited since shorter plug lengths are desired for small-bore columns to take advantage of their potentially greater spatial resolution. The reduction in analyte reduces the amount of analyte available for detection. This effective reduction in instrument sensitivity has limited commercialization of small-bore columns, despite advantages in throughput and resolution.
Cold traps promise to address both the needs for reduced plug length and increased effective sensitivity for small-bore columns. A cold trap can be used to concentrate and focus an analyte on a cooled stationary phase prior to introduction to the column. Typically, a metal or glass cold trap is coated or packed with absorbent organic polymer. The packing slowly absorbs a flowed-through analyte, which is thereby concentrated. The cold trap can then be heated rapidly so that the analyte is flash-volatilized to produce a sharp plug of analyte for introduction into the column. In addition to improving resolution, the cold trap concentrates the components so that they are more readily detected, effectively increasing instrument sensitivity.
So that the focal region is well defined, the focal zone must be much more absorptive than adjacent zones of the cold trap. Even without organic absorptive material to trap it, analyte can condense outside the focussing zone, impairing the definition of the analyte plug to be injected onto the column. Heating the cold trap away from the focussing zone can prevent this condensation. Such heating can be achieved by interfacing the trap with a hot manifold so that the cold-trap ends are heated. In addition, the focussing zone can be cooled to increase its retentiveness. A sharply defined hot-cold-hot temperature gradient is preferred to minimize plug length.
Once the analyte is concentrated in the cold trap, the cold trap can be heated to release the trapped analyte into the column. To minimize plug spreading during release, the heating should be very rapid, e.g., from below ambient to hundreds of degrees Celsius above ambient in tens of milliseconds or less. A trap can be resistively heated by passing a large current therethrough to vaporize the analyte. For resistive heating, the cold trap must be conductive, thus excluding insulators such as glass as the trap wall material. Since a trap typically represents a low resistance, e.g., on the order of 10 milliohms, delivering the required current, e.g., hundreds of amperes, to provide the rapid heating has been problematic.
A system using transformers to generate large currents using 60 Hz power sources is disclosed by B. A. Ewels and Robert D. Sachs in "Electrically Heated Cold Trap Inlet System for High-Speed Chromatography", American Chemical Society, 1985, pp. 2274-2279. A transformer connected to a 208 Volt 60 Hz power source provides an initial rapid heating of a stainless-steel cold trap. Despite the great need for a gas chromatography system with a resistively heated cold trap, this system has not given rise to a commercially successful system. The lack of success can be attributed to the compromises required to draw sufficient power to heat the trap rapidly. In the present case, some of the compromises are: 1) a requirement for a higher-than-standard line voltage; 2) a large and expensive transformer; and 3) a requirement for line power.
An alternative gas-chromatography injector discharges a bank of capacitors through a cold trap, as disclosed by Mark A. Klemp, Michael L. Akard, and Robert D. Sachs, in "Cryofocusing Inlet with Reverse Flow Sample Collection for Gas Chromatography", American Chemical Society, 1993, pp. 2516-2521. The problem of extracting high power from an AC source is avoided since the AC power source is used merely to charge the capacitors. However, the cold trap represents only a small fraction of the circuit resistance of the discharge circuit (which includes silicon-rectifier switches). Thus, only a small fraction of the energy delivered by the capacitors is converted to heat at the cold trap. As a result, the capacitors must be able to hold many times the energy required for heating the cold trap. The bulk and expense of the required capacitors are impediments to a commercial realization.
A variation of the foregoing injector employs a thin-walled cold trap to increase trap resistance and reduce the required capacitance. However, the thin trap is unacceptably fragile and is prone to breakage. Another approach to reducing power requirements is to use a more resistive trap wall material. Nickel is a preferred trap wall material because of its chemical inertness. Stainless steel, used in the Ewel et al. system described above, has greater resistance. However, stainless steel is more reactive chemically than nickel, so use of stainless steel unacceptably limits the types of analytes that can be analyzed.
In the foregoing systems, the electrical conductors delivering current to the cold traps are thermally conductive; thus, they can conduct heat away from the trap ends; this can impair the sharpness of the thermal gradient along the trap required for establishing a short analyte plug. The conductors can be heated, but this can have adverse consequences as well. If the conductors are copper, the heat can oxidize them, increasing their resistivity. Other materials, e.g., nickel, oxidize less readily, but are more resistive to begin with. Attempts to use a transformer secondary as the conductor have been hindered by the current requirements for the primary and the size required of the transformer core.
Thus a long-standing need has existed for a commercially practicable cold-trap focusing-injector for gas chromatography, i.e., a system for introducing analyte into a gas-chromatography column that uses a cold-trap to concentrate the sample prior to injection. Such an injector would address the sensitivity limitations of prior art small-bore columns and help realize their potential for speedy, high-resolution, analyses.